Due to the recent developments of tissue engineering and regenerative medicine, methods capable of treating damaged tissues and organs are being developed in a different manner from the conventional methods, and the cell therapy by mesenchymal stem cells is receiving most attention. Stem cells refer to cells that can proliferate indefinitely in an undifferentiated state as well as differentiating to have a specialized function and shape under specific environments and conditions. Examples of stem cells are: embryonic stem cells derived from human embryos; and adult stem cells, such as bone marrow cells that constantly generate blood cells. Embryonic stem cells can differentiate into all the cells and tissues constituting the human body, but the use thereof is limited for ethical reasons. Adult stem cells, on the other hand, are extracted from the umbilical cord blood or the bone marrow and blood of fully grown adults, enable the differentiation into specific tissues and organs after in vivo transplantation, and have the differentiation flexibility to transdifferentiate into cells of other tissues different from characteristics of original cells. Adult cells are widely used in tissue engineering without ethical limitations. In recent years, various attempts and early clinical trials are on the way in the medical field for the regeneration and replacement of tissues or organs of patients by growing stem cells and then differentiating the stem cells into specific cells. Mesenchymal stem cells are one type of adult stem cells present in various organs or blood of the body after the development and are a cell source that is easy to maintain and has no ethical problems. The mesenchymal stem cells are currently the most notable stem cell in the regenerative medicine field, but have a drawback in that the mesenchymal stem cells have limits in the in vitro subculture and differentiation potency compared with embryonic stem cells.
Studies on humans and animals have already confirmed that bone marrow-derived stem cells out of adult stem cells differentiate into osteogenic cells (Friedenstein A. J. et al., Transplantation., 6:230-247, 1968), and recent studies have progressed methods for culturing stem cells isolated from the bone marrow to differentiate the stem cells into osteoblasts, and there is an increasing possibility of clinical application using the methods (Ohgushi H. et al., J. Biomed Mater Res., 48:913-927, 1999). Recently, methods for differentiation into osteocytes from mesenchymal stem cells have dominantly been studied.
On the other hand, methods for the culture and differentiation of cells in a two-dimensional well plate are currently most widely used in the differentiation in stem cells. However, there are recent paper reports that two-dimensional (monolayer) cell culture lowers cell functions and significantly changes morphology compared with three-dimensional cell culture (Proc. Natl. Acad. Sci. USA, 100: 1943-1948, 2003; Cell, 111: 923-925, 2002; Cancer Cell 2: 205-216, 2002). The cell culture and differentiation in a manner of adversely affecting the state of cells as described above causes difficulty in differentiation and takes a long time. In order to overcome the drawbacks of such cell culture, Korean Patent No. 10-0733914 discloses a three-dimensional microcellular culture system characterized in that cells are present in a three-dimensional gel, but the gel needs to be dissolved in order to separate the cells present in the gel after the culture or differentiation of stem cells, causing severe cell damage.